In the field of satellite telecommunications, it is necessary to employ a beamforming antenna making it possible to cover a vast territory, such as Europe for example, with a very large number of fine beams having an angular aperture of for example less than 0.2°, and with good overlap of the beams.
A first architecture of beamforming antenna, called a reflector antenna with a focal array, consists in using an array of sources associated with a reflector, for example parabolic, the array of sources, called a focal array, being placed in a focal plane situated at the focus of the reflector. In reception, the reflector reflects an incident plane wave received and focuses it in the focal plane of the reflector on the focal array. Depending on the direction of arrival of the incident plane wave on the reflector, its focusing by the reflector is carried out at various points of the focal plane. The reflector therefore makes it possible to concentrate the energy of the incident signals received on a reduced zone of the focal array, this zone depending on the direction of arrival of the incident signal. The synthesis of a beam corresponding to a particular direction can therefore be carried out on the basis of a reduced number of preselected sources of the focal array, typically of the order of seven sources for a focal array comprising for example of the order of two hundred sources. The sources selected for the synthesis of a beam are different from one beam to another and selected according to the direction of arrival of the incident signals on the reflector. For the synthesis of a beam, a beamformer combines all the signals focused on the sources selected dedicated to this beam. The number of sources dedicated to a beam being small, this type of antenna exhibits the advantage of operating with a beamformer of reduced complexity which poses no major problem in respect of its production even when the number of beams increases appreciably, for example for 400 beams. However in case of loss of a source, for example subsequent to a fault with a signal amplifier positioned at the output of this source, the corresponding beam will be greatly impaired. To avoid the loss of a source, it is therefore necessary to double the number of amplifiers positioned at the output of each source as well as all the corresponding electronic control pathways. This increases the complexity and bulk of the antenna.
A second architecture of beamforming antenna, called a phased array antenna, consists in using an array of direct-radiation radiating sources in which all the sources participate in the synthesis of each of the beams, the synthesis of each beam being carried out by a beamformer by applying a phase shift matrix at the output of the array of radiating sources so as to compensate for the radiation delay of the sources with respect to one another for each direction of radiation of the array of radiating sources. Consequently, all the beams are formed by the whole set of sources, only the delay law applied to each source changes from beam to beam. This architecture exhibits the advantage of lesser sensitivity of the antenna in case of loss of sources and makes it possible to decrease the number of amplification pathways by a factor of two but exhibits the drawback of a beamformer which is very complex to produce, or indeed impossible to produce currently when the number of beams to be synthesized is very significant. Indeed, to synthesize for example a beam with an array of 300 radiating sources, the beamformer must combine the 300 RF signals at the output of each source. To synthesize 100 beams with an array of 300 radiating sources, this combining must be carried out 100 times. The corresponding phase shift matrices are therefore very voluminous and cannot be produced with RF circuits. Consequently, this type of antenna currently exists only for a limited number of beams and sources, such as for example 6 beams and 64 sources.
It is possible to carry out the synthesis of a large number of beams and to obtain a large number of spots by using digital beamforming. Accordingly, the RF signals are converted at the level of each source into digital signals before being applied as input to the digital beamformer. However, this solution requires the implanting of frequency transposition devices and analog-digital converters at the level of each source, thereby increasing the complexity, mass, volume and consumption of the antenna and is not acceptable for use in the field of multimedia telecommunications.
A third architecture of multiple-beam forming antenna, consists in using a phased array which comprises sources of small size and is magnified by an optical system comprising one or more reflectors. This architecture can be called an imaging array antenna, since globally the focal array retains the same characteristics as a direct-radiation phased array, the synthesis of a spot being carried out by almost the entirety of the sources.
A first configuration of imaging array antenna comprises two parabolic reflectors, main and secondary, having the same focus and a phased array. The main parabolic reflector is of large size, the secondary parabolic reflector is of smaller size, the phased array placed in front of the secondary reflector comprises sources of reduced size. The behavior of this antenna is similar to that of the direct-radiation phased array antenna but exhibits the advantage of increasing the size of the radiating aperture of the antenna with respect to a direct-radiation phased array antenna, with a magnification factor defined by the ratio of the diameters of the two reflectors, thereby making it possible to decrease the size of the sources of the phased array and therefore the size of the beams. Its main drawback resides in the complexity of the beamformer associated with the phased array since, as in the case of the direct-radiation phased array antenna, the whole set of sources participates in the contribution of the whole set of beams.
A second configuration of imaging array antenna comprises a single parabolic reflector and a defocused phased array placed in front of the reflector. This configuration exhibits a magnification factor of the radiating aperture of the antenna with respect to a direct-radiation phased array antenna, equal to the ratio between the focal length of the parabolic reflector and the distance at which the array has been defocused. In this configuration, most of the sources participate in an identical manner in the contribution of the whole set of beams, but the operation of the phased array is a little different from that of a direct-radiation phased array, or from that of the phased array associated with the first imaging array antenna configuration. Unlike these two types of phased arrays which emit a plane wave, the defocused array associated with an imaging array antenna configuration with a single reflector emits a spherical wave, which is converted into a plane wave by the main reflector.
The two imaging array antenna configurations exhibit two major drawbacks. Because of the remoteness of the phased array from the focus of the reflector or reflectors, they induce aberrations. Indeed, the phase distribution over the radiating aperture associated with the main reflector is affected by a spatial phase distortion which is all the more significant as the signal beam is squinted. These phase distortions are manifested by a degradation of the radiated beam and must be compensated for by modifying the feed law for the phased array. The two imaging array antenna configurations also exhibit a second drawback stemming from the variation of the size of the radiating aperture as a function of the squinting of the beam and due to the fact that the surface area of interception of a beam emitted by the phased array varies as a function of the squint angle. To obtain a radiating aperture of identical size, it is then necessary to adjust the size of the phased array as a function of the squint angle.
On account of these various drawbacks, an orthogonal-beam former, developed for a direct-radiation phased array, is not optimal if it is used for imaging array antennas. The beamformer must be designed in association with the optical system of the antenna, that is to say with the reflector or reflectors, this being impossible with existing beamformers for which the beamformer is designed independently of the antenna reflectors.
A fourth architecture of beamforming antenna comprises a quasi-optical beamformer in which a signal emitted by a set of input ports is guided between two parallel metallic plates toward an output port. The propagation of the signal emitted is interrupted by a reflector wall which reflects it and focuses it on the output port.
Two different configurations of quasi-optical beamformer exist. According to a first configuration, the input and output ports are situated in one and the same propagation medium defined between two parallel plates, the propagation medium being able to comprise a dielectric. In this case, the input and output ports are distributed along two distinct orthogonal axes and the reflector wall is illuminated with an angle of offset so that it transmits the entirety of the signal from the input ports to one, or several, output port or output ports.
According to a second configuration, called a pill-box structure, the input and output ports are situated in two different superposed propagation media, each propagation medium being defined between two parallel metallic plates. The two substrate layers constituting the two propagation media are coupled by an internal reflector wall extending transversely with respect to the planes of the layers. The first substrate layer, for example the lower layer, comprises at least one RF energy source placed at the focus of the internal reflector. The output ports are situated in the second substrate layer. To improve the transition of the waves between the two substrate layers, document FR 2 944 153 describes the making of coupling slots extending along the internal reflector.
In these two configurations, in emission, the energy source placed at the focus of the internal reflector emits a cylindrical incident wave guided in the tri-plate propagation medium. The cylindrical incident wave is reflected by the internal reflector which transforms it into a plane wave. The reflected plane wave is thereafter conveyed by waveguides up to an array of radiating slots. The energy is then radiated by radiating slots in the form of a beam. The formation of the beam radiated by the antenna is carried out in a natural manner by simple guidance of the wave in the substrate layer, or in the two substrate layers, and by way of the quasi-optical transition means consisting of the internal reflector and optionally the coupling slots. The displacement of the source in the plane of the focus of the reflector generates wavefronts corresponding to given directions of propagation. A scan and a squinting of the beam in elevation, in a plane perpendicular to the plane of the antenna, is obtained by switching various sources. However, given that the sources are situated in one and the same plane, the squinting of the beam cannot be carried out in all directions in space but only in a single plane and no azimuthal beamforming is possible.